Figure 2-1. Basic immunoglobulin structure
(V indicates variable region; C indicates constant region).
CHARACTERISTICS OF IMMUNOGLOBULIN CLASSES
a. IgG. In normal human adults, IgG constitutes approximately 75% of total
serum immunoglobulins. It is the best known and most fully studied of the
immunoglobulins. The molecule is made up of one basic structural unit known as a
monomer, consisting of two heavy chains and two light chains (Figure 2-1). IgG is the
only class of immunoglobulin that can cross the placenta, and it is responsible for the
protection of the newborn during the first few months of life.
b. IgM. This high-molecular-weight macromolecule constitutes approximately
10% of the normal serum immunoglobulins. It exists as a pentamer consisting of ten
heavy chains and ten light chains joined together by a J chain (Figure 2-2).
c. IgA. IgA represents approximately 15% of the serum immunoglobulins. It
normally exists in serum both in monomeric and polymeric forms. The IgA dimer
consists of two monomeric units (Figure 2-3). It is the predominant immunoglobulin
class found in body secretions.
d. IgD. The IgD molecule is a monomer and is normally present in serum in
trace amounts. Its main function has not been determined.
e. IgE. IgE comprises only 0.004% of the total serum immunoglobulins. It
normally exists in monomeric form.